1. Field of the Invention
The invention relates to a two stage catalytic cracking process comprising both a fluidized catalytic cracking zone and an ebullated catalyst bed hydrocracking zone. More particularly, the invention relates to the serial catalytic cracking of a heavy cycle gas oil fraction boiling in the range of 600.degree. F. to 1050.degree. F. to yield a liquid fuel and lighter boiling range fraction.
2. Description of Other Relevant Methods in the Field
The ebullated bed process comprises the passing of concurrently flowing streams of liquids or slurries of liquids and solids and gas through a vertically cylindrical vessel containing catalyst. The catalyst is maintained in random motion in the liquid and has a gross volume dispersed through the liquid greater than the volume of the catalyst when stationary. This technology has found commercial application in the upgrading of heavy liquid hydrocarbons or converting coal to synthetic oils.
The process is generally described in U.S. Pat. No. Re. 25,770 to Johanson incorporated herein by reference. A mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into random motion as the liquid and gas flow upwardly through the bed. The random catalyst motion is controlled by recycle liquid flow so that at steady state, the bulk of the catalyst does not rise above a definable level in the reactor. Vapors along with the liquid which is being hydrogenated, pass through that upper level of catalyst particles into a substantially catalyst free zone and are removed at the upper portion of the reactor.
In an ebullated bed process the substantial amounts of hydrogen gas and light hydrocarbon vapors present rise through the reaction zone into the catalyst free zone. Liquid is both recycled to the bottom of the reactor and removed from the reactor as product from the catalyst free zone. The liquid recycle stream is degassed and passed through the recycle conduit to the recycle pump suction. The recycle pump (ebullation pump) maintains the expansion (ebullation) and random motion of catalyst particles at a constant and stable level.
A number of fluid catalytic cracking processes are known in the art. State of the art commercial catalytic cracking catalysts for these processes are highly active and possess high selectivity for conversion of selected hydrocarbon charge stocks to desired products. With such active catalysts it is generally preferable to conduct catalytic cracking reactions in a dilute phase transport type reaction system with a relatively short period of contact between the catalyst and the hydrocarbon feedstock, e.g. 0.2 to 10 seconds.
The control of short contact times, optimum for state of the art catalysts in dense phase fluidized bed reactors is not feasible. Consequently, catalytic cracking systems have been developed in which the primary cracking reaction is carried out in a transfer line or riser reactor. In such systems, the catalyst is dispersed in the hydrocarbon feedstock and passed through an elongated reaction zone at relatively high velocity. In transfer line reactor systems, vaporized hydrocarbon cracking feedstock acts as a carrier for the catalyst. In a typical upflow riser reactor, the hydrocarbon vapors move with sufficient velocity to maintain the catalyst particles in suspension with a minimum of back mixing of the catalyst particles with the gaseous carrier. Thus development of improved fluid catalytic cracking catalysts has led to the development and utilization of reactors in which the reaction is carried out with the solid catalyst particles in a relatively dilute phase with the catalyst dispersed or suspended in hydrocarbon vapors undergoing reaction, i.e., cracking.
With such riser or transfer line reactors, the catalyst and hydrocarbon mixture passes from the transfer line reactor into a first separation zone in which hydrocarbons vapors are separated from the catalyst. The catalyst particles are then passed into a second separation zone, usually a dense phase fluidized bed stripping zone wherein further separation of hydrocarbons from the catalyst takes place by stripping the catalyst with steam. After separation of hydrocarbons from the catalyst, the catalyst is introduced into a regeneration zone where carbonaceous residues are removed by burning with air or other oxygen-containing gas. After regeneration, hot catalyst from the regeneration zone is reintroduced into the transfer line reactor into contact with fresh hydrocarbon feed.
U.S. Pat. No. 3,905,892 to A. A. Gregoli teaches a process for hydrocracking a high sulfur vacuum residual oil fraction. The fraction is passed to a high temperature, high pressure ebullated bed hydrocracking reaction zone. The reaction zone effluent is fractionated into three fractions comprising (1) a 650.degree. F..sup.- fraction (light ends and middle distillates), (2) a 650.degree. F. to 975.degree. F. gas oil fraction and (3) a 975.degree. F..sup.+ heavy residual vacuum bottoms. The 650.degree. F. to 975.degree. F. gas oil fraction is passed to processing units such as a fluid catalytic cracking unit. The vacuum bottoms is deasphalted and the heavy gas oil fraction recycled to extinction in a fluid catalytic cracker described in the Abstract of the Gregoli patent.
U.S. Pat. No. 3,681,231 to S. B. Alpert et al teaches an ebullated bed process wherein a petroleum residuum feedstock containing at least 25 vol % boiling above 975.degree. F. is blended with an aromatic diluent boiling within the range of 700.degree. F. to 1000.degree. F. and API gravity less than 16.degree.. The aromatic diluent is blended in a ratio of 20 to 70 vol %, preferably 20 to 40 vol % diluent based on feed.
Aromatic diluents include decant oils from fluid catalytic cracking processes, syntower bottoms from Thermofor catalytic cracking operations, heavy coker gas oils, cycle oils from cracking operations and anthracene oil obtained from the destructive distillation of coal. It is stated that the 700.degree. F. to 1000.degree. F. gas oil generated in the process will in certain cases fall within the range of gravity and characterization factor and can serve as the aromatic feed diluent.
U.S. Pat. No. 3,412,010 to S. B. Alpert et al teaches an ebullated bed process wherein a petroleum residuum containing at least 25 vol % boiling above 975.degree. F. is mixed with a recycle 680.degree. F. to 975.degree. F. fraction and passed to the ebullated reaction zone. It was found that the recycle of a 680.degree. F. to 975.degree. F. heavy gas oil resulted in a substantial lower yield of heavy gas oil in the 680.degree. F. to 975.degree. F. range and an increased yield of naphtha and furnace oil. Substantial improvement in operability was achieved as a result of reduction in asphaltenic precipitates.
U.S. Pat. No. 4,523,987 to J. E. Penick teaches a feed mixing technique for fluidized catalytic cracking of a hydrocarbon oil. The product stream of the catalytic cracking is fractionated into a series of products, including gas, gasoline, light gas oil and heavy cycle gas oil. A portion of the heavy cycle gas oil is recycled to the reactor vessel and mixed with fresh feed.